Expected Residence Time Research Articles

Stochastic Prediction of Drought Class Transitions

This paper aims at the stochastic characterization of droughts applying Markov chains modeling to drought class transitions derived from SPI time series. Several sites in Southern Portugal having updated data on precipitation available were considered. The drought class probabilities, the expected residence time in each class of severity, the expected time for the transition between drought classes and the drought severity class predictions 1, 2, or 3 months ahead have been obtained. Those predictions are then compared with observed drought classes for the recent drought periods of 2003–2006. In addition, the estimation of the cumulated precipitation deficits, amount of monthly precipitation needed to decrease drought severity, and foreseen SPI values depending on different precipitation scenarios are also presented as complementing the prediction of drought class transitions.

Analysis of the residence time distribution in large flotation machines

Residence time in a single flotation machine is one of the most important parameters necessary to properly design a flotation circuit. The mean residence time for any particular cell design is usually determined from residence time distribution (RTD) experimental results. The comparison of the measured mean residence time with the expected residence time, as well as analysis of the shape of the RTD curve, give the most useful and valid information about the mixing properties in the cell. Experimental RTD results for three large flotation cells (Dorr-Oliver, Outokumpu and WEMCO ®) discussed in this publication were released by the Chuquicamata Division of CODELCO as a part of the competitive rougher flotation test report (CODELCO (National Copper Corporation of Chile), Chuquicamata Division, 2000, Project IM2, No. 19/99; Final Report of the Evaluation of Large Volume Flotation Cells). Evaluation of the results by CODELCO as well as by authors is provided. “Approximation of the theoretical curve of a perfect mixer,” used by CODELCO as a base for cell categorization is analyzed and critiqued after comparing to the criteria used by many authorities in the field. Conclusions are drawn based on the multi-parameter flow model, showing existence of stagnant volume of 45% in Outokumpu, 35% in Dorr-Oliver, and 7% in WEMCO ® flotation cell volumes.

Particle clustering in down flow reactors

This study describes a new optical sensor, the CREC-GS-Optiprobe. This sensor can be used in gas–solid reactors for characterizing clustering phenomena with minimum intrusion effects. Theoretical considerations allow to anticipate a narrow focal region at 4.5 mm away from the sensor tip. These theoretical predictions are confirmed with calibrations and experimental testings of the novel sensor. It is the goal of the present study to offer evidence, using this novel CREC-GS-Optiprobe, of cluster formation in down flow reactors. It is demonstrated through experimental studies that in down flow systems ( d p=65 μm FCC particles, solid fluxes=3–7 kg m −2 s) that catalyst evolves, as strings of particles, with a relatively large size distribution: average cluster sizes are comprised between 2 and 6 d p with most of these strings having 3.5 d p. Using theoretical considerations and solving the equations of motion, it is proven that hydrodynamic interactions between particles lead, in down flow reactors, to formation of particle agglomerates (clusters). Key parameters in this process are particle approaching times and particle approaching velocities. It is shown that particle agglomeration is a very rapid process taking place in downer units in a time scale much smaller than the few seconds expected residence times.

Population Ecology of the Kamehameha Butterfly (Lepidoptera: Nymphalidae)

Mark-recapture studies of the endemic Hawaiian Kamehameha butterfly, Vanessa tameamea Eschscholtz, were conducted at five sites on Tantalus Mountain, Oahu, where adults fed on sap oozing from injured Acacia koa Gray (Mimosaceae) trees. Males outnumbered females at the sap fluxes by 4.5 to 1, but females accounted for 58% of 137 specimens collected at diverse sites during the last century. Thus, we infer that the proportion of time spent at sap fluxes was greater for males than females. Males showed high site fidelity; none of 51 recaptures of males were transfers between sites. Males' expected residence time was 16 d. The estimated number of males at the most intensively studied site varied from 13 to 36 during a 35-d period. On the island of Hawaii, a search of 16,360 leaves of Pipturus albidus (Hooker & Arnott) Gray (Urticaceae), the primary larval host plant, yielded one larva of V. tameamea . Although habitat destruction may have reduced the distribution of V. tameamea , results suggest that food for larvae is not a limiting resource for extant populations.

A High Temperature, High Pressure Reaction-Screening Apparatus.

This short note describes an apparatus that has been designed and constructed to allow assessment of the extent of chemical decomposition of fluids and fluid mixtures under high temperature, high pressure conditions. The apparatus is used to screen fluid systems prior to PVT (pressure-volume-temperature) or VLE (vapor-liquid equilibrium) experiments under severe conditions. For a predetermined residence time, the fluids are maintained at the temperature and pressure at which the PVT or VLE experiment will be conducted. The residence time in the reactor is comparable to the expected residence time in the PVT or VLE apparatus. Samples of fluid are withdrawn directly at regular intervals for analysis by gas chromatography, or collected In a sampling vessel for more extensive analysis.

A Markovian model for ecosystem flow analysis

Nutrient flow through ecosystems is modeled as a discrete Markov chain whose transition probabilities are stationary or time inhomogenous according to whether a steady state or dynamic ecosystem is modeled, respectively. Expected residence time and number of intercompartmental transfers for a nutrient within a set of compartments are derived. Variances of these random variables are also considered. A measure for ecosystem resource recycling is given as a weighted sum of probabilities.

Hydrogeologic Studies at a Subsurface Radioactive-Waste Management Site in West-Central Canada: ABSTRACT

Nearly all radioactive wastes produced in Canada are stored or disposed of at 2 waste-management sites in shallow Quaternary deposits in south-central Canada. The oldest site and the one which has received most of Canada's radioactive waste is at the Chalk River Nuclear Laboratories (CRNL) in east-central Ontario. The newest site is at the Whiteshell Nuclear Research Establishment (WNRE) in southeastern Manitoba. Both waste-management sites are operated by Atomic Energy of Canada Limited. Hydrogeologic studies were conducted at WNRE to predict the subsurface behavior of radioactive contaminants, which at some future time possibly could enter the groundwater zone. The site is composed of Quaternary deposits of clayey till above a sandy artesian aquifer which overlies the Precambrian bedrock. The bedrock is 50 ft below ground surface. At present there is no significant contamination of the subsurface hydrologic environment. Hydrogeologic information is being used to facilitate the design and operation of the waste-management facilities and to assess long-term storage and disposal capabilities of the area. The hydrogeologic investigations involved 3 main parameters. These are (1) expected residence times of radionuclides which may enter the groundwater flow system, (2) anticipated travel paths and discharge processes, and (3) suitability of the hydrogeologic environment for physical manipulation to achieve greater containment capabilities. Hydrogeologic studies conducted during the past 5 years have involved field and laboratory techniques such as geologic test drilling, mapping of hydraulic head distributions using wells and piezometers, field permeability tests using single well response tests and long- and short-term pumping tests, mapping of natural hydrochemical patterns in the groundwater zone, tritium tracer experiments, groundwater age dating using C14, and mathematical modeling using digital-simulation programs. Comparisons of the results from the studies indicate that we have attained a reasonable level of predictability in our understanding of the hydrogeologic environment in the area. End_of_Article - Last_Page 1593------------

On the optimum transfer coefficient at an exchange surface in a gas-fluidized bed

A surface renewal model which uses a generalized gamma distribution to represent the expected residence times of particles is developed to describe heat transfer from a transport surface to a gas fluidized bed of solid particles. The observed maximum heat transfer coefficient value at a particular gas mass velocity is predicted using photographic residence time measurements. The average heat transfer coefficient depends on the expected heat pick up of a single particle entering the effective transfer region and the average flux of particles entering the region. Nusselt numbers calculated from the model equation are generally lower than experimental values. The particle flux depends on the bubble frequency which may be related to bed properties. The model may be used with existing heat transfer data to predict the effect of particle and gas properties on surface to bed coefficients and the optimum value for a bed of known particle properties.